Xplor3.843 map.

Gerard 'CD' Kleywegt GERARD at XRAY.BMC.UU.SE
Wed Nov 20 14:09:15 EST 1996

sigma levels of difference maps are *MEANINGLESS*
consider this:
at the beginning of your refinement, your model is
incomplete and contains errors -> lot of real signal
in the difference map and even a "1.5 sigma" peak may
be highly significant
at the end of refinement, by definition your difference
map is essentially flat, and any variation is random
noise; nevertheless there may still be "3 sgima" peaks
of course, only they don't mean anything
so, contour your difference map at some Electron/A3
level rather than at "N sigma" levels

of course your ligand could still be partly occupied;
you could see how the maps (and temp. factors) look
if you put it at half occupancy (if you have very
high resolution data you can also try to refine


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